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W. S.;Allard, D. W. Sei. Total Environ. 1982,23,345-360. Eatough, D. J.; Hansen, L. D. Adu. Environ. Sci. Technol.
(17) Hoffman, G. R. Mutat. Res. 1980, 75, 63-129.
12, 1983, 221-268.
Eatough, D. J.; Hansen, L. D. Sci. Total Enuiron. 1984,36, 319-328.
Slanina,J.; Schoonebeck, C. A. M.; Klockow, D.; Niessner, R. T h e Determination of Sulfuric Acid and Ammonium Sulfate by Means of a Computer Controlled Thermo-Denuder System; in press. Tan, E.-L.; Brimer, P. A.; Schenley, R. L.; Shie, A. W. J. Toricol. Environ. Health 1983, 11, 373-379.
Received for review March 9,1984. Revised manuscript received March 17,1986. Accepted March 31,1986. Although the research described i n this article has been funded wholly or in part by the U S . Environmental Protection Agency through Assistance Agreement CR-810335-01 t o Brigham Young University, it has not been subjected to Agency review and therefore does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
Determination of Gas-Phase Dimethyl Sulfate and Monomethyl Hydrogen Sulfate+ Lee D. Hansen, Vernon F. White, and Delbert J. Eatough" Department of Chemistry and Thermochemical Institute, Brigham Young University, Provo, Utah 84602
Analytical techniques have been developed for the collection and determination of gas-phase dimethyl sulfate and monomethyl hydrogen sulfate. The techniques include the collection of the gas-phase species in diffusion denuders that are selective for monomethyl hydrogen sulfate or that collect both dimethyl sulfate and monomethyl hydrogen sulfate. Dimethyl sulfate and monomethyl hydrogen sulfate are also collected in filter pack adsorbent bed combinations which allow for the separate determination of both dimethyl sulfate and monomethyl hydrogen sulfate or for their joint collection and determination. The sampling and analysis techniques thus allow for the determination of both di- and monomethyl sulfates each by two independent procedures with a detection limit of 2-10 nmol by ion chromatography. Data obtained with the adsorbent bed sampling techniques indicate that gas-phase dimethyl sulfate is present at very low concentrations in the flue line of a coal-fired steam plant.
Introduction We have previously reported the presence of dimethyl and monomethyl sulfate in airborne particulate matter and in fly ash from coal- and oil-fired power plants (1-5). These previous studies (1,3,4) also indicate that dimethyl sulfate in particles in the plumes of both oil- and coal-fired power plants increases with plume age. Because of the known mutagenicity of dimethyl sulfate (6, 7), sensitive analytical techniques are needed to determine gas-phase dimethyl sulfate and monomethyl hydrogen sulfate. The collection of dimethyl sulfate and monomethyl hydrogen sulfate from the gas phase may be complicated because of reactions such as (CH30)2S02 + HzO = (CHSO)SO3H + CHSOH (1) (CH30)S03H+ H 2 0 = HzS04+ CH30H (2) 2(CH30)S03H= (CH30)2S02+ HzS04 (3) Since each of these reactions is expected to occur in the atmosphere, more than one analytical technique for the collection and determination of gas-phase dimethyl sulfate and monomethyl hydrogen sulfate is desired to ensure that these reactions are avoided during sample collection and analysis. This report describes the development and validation of procedures for the collection and determination of both 'Contribution No. 362 from the Thermochemical Institute. 872
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dimethyl sulfate and monomethyl hydrogen sulfate in the gas phase using a variety of diffusion denuder tube and sorbent bed collection systems.
Experimental Section Preparation of Dimethyl Sulfate and Monomethyl Hydrogen Sulfate Standards. Potassium monomethyl sulfate is available (Eastman) in reagent grade. Determination by ion chromatography (5) of both sulfate and monomethyl sulfate in solutions of potassium monomethyl sulfate prepared by weight indicated that the purity of the commerical sample used in this study was 94% by weight. Aqueous solutions of the potassium monomethyl sulfate were stable for at least 6.months with no measurable change in composition. Samples of dimethyl sulfate were prepared by vacuum distillation of the commercial material (Mallinckrodt) at 80 "C. The concentrations of both dimethyl sulfate and monomethyl sulfate in freshly prepared aqueous solutions of the distilled sample were determined by ion chromatography ( 3 , 5 ) . The distilled samples had a purity of about 98% by weight, with the impurity being monomethyl sulfate. The freshly distilled samples could be stored at room temperature for a few weeks before measurable decomposition occurred. Monomethyl hydrogen sulfate is not available commercially. Monomethyl hydrogen sulfate was prepared from the potassium monomethyl sulfate salt by cation exchange of K+ for Hf using Dowex ion-exchange resin in the acid form. Twenty-five milliliters of the resin was packed in a column and thoroughly washed with anhydrous methanol. A saturated solution of K(CH30)S03in methanol was then put on the column and eluted with methanol, the (CH30)S03Hfrom the column being detected by the measurement of acidity. The solvent was then rapidly rwoved in a Rotovac at room temperature with no heating of the sample. The resulting viscous liquid was stored at -80 "C until used. Ion chromatographicanalysis of the samples indicated that the purity of monomethyl hydrogen sulfate prepared by this procedure varied from 70% to 95%. The impurities were dimethyl sulfate and sulfuric acid. The monomethyl hydrogen sulfate slowly disproportionated according to eq 3 even at -80 "C. At room temperature, the half-life of reaction 3 for the liquid monomethyl hydrogen sulfate was about 8 h. As a result, samples of liquid monomethyl hydrogen sulfate used to generate (CH30)S03H(g)were used immediately after removal from the -80 O C storage chamber.
0013-936X/86/0920-0872$01.50/0
0 1986 American Chemical Society
Gas-phase dimethyl sulfate and monomethyl hydrogen sulfate were both generated from the corresponding liquid by passing pure N2 or Ar over the liquid to sweep away the vapor in equilibrium with the liquid. In all experiments used to evaluate the collection of gas-phase methyl sulfates by denuders or adsorbent beds, a filter was placed between the gas-phase generator and the collector to remove any aerosol present. Denuders. Field experiments done in several urban areas by Durham et al. (8, 9) have shown that a nylon denuder collects a gas which, after storage of the denuder for several weeks to months, produces sulfate upon ion chromatographic analysis of aqueous extracts of the denuder. The sulfate deposition patterns indicate that the diffusion coefficient, Do,of the collected gas is the same as that expected for dimethyl sulfate, Do = 0.073 cm2/s, or monomethyl hydrogen sulfate, Do= 0.081 cm2/s (8). It was shown that the gas was not SO2 or H2S. Most significantly, the concentration of the gaseous sulfur compound inferred from these denuder measurements was usually comparable to the concentrations of particulate sulfate. Similar concentrations of an unidentified gasphase organic sulfur compound in urban atmospheres have been inferred from experiments with an activated carbon denuder (IO). We have studied the collection of gas-phase dimethyl sulfate, monomethyl hydrogen sulfate, combinations of dimethyl sulfate and monomethyl hydrogen sulfate, and SOzby nylon denuders. In the course of these experiments, we found that the paper dividers supplied with the nylon strips were selective for the collection of monomethyl hydrogen sulfate. Therefore, the collection of dimethyl sulfate, monomethyl hydrogen sulfate, and SO2 by the paper dividers was also studied. Diffusion-denuder tubes were made by placing rolled tubes of nylon-membrane sheets (Membrana) or the paper sheets inside Teflon carrier tubes. The inside diameter of the Teflon tube was either 0.78 or 0.70 cm. Six 10-cm long nylon or paper tubes were used in each denuder. Laboratory Studies with the Denuders. The collection pattern of dimethyl sulfate, monomethyl hydrogen sulfate, and SO2 by both the nylon and paper denuders in the absence of particles was determined. The gas-phase dimethyl sulfate and monomethyl hydrogen sulfate were generated by passing N2 or Ar over the pure liquids or a mixture of the two liquids. In some cases, the collection of dimethyl sulfate and monomethyl hydrogen sulfate by the paper denuder in the presence of H 2 0 vapor was also studied. In these experiments, the gas-phase alkyl sulfate was mixed with an air stream of equal flow rate containing H20(g)obtained by flowing air over water. In all cases, any particles in the air stream were removed by using an acid-washed quartz filter, and the resulting gas was pulled through the denuder at a flow rate of 1.5 L/min. After the denuder, dimethyl sulfate or monomethyl sulfate was collected with two nylon filters or an aqueous bubbler. A basic, aqueous bubbler was used to collect SOp. The quartz prefilter, the 10-cm denuder sections, and the nylon filters were all immediately extracted with water at 25 " C in an ultrasonic bath for 15 min. The extracted monomethyl hydrogen sulfates, sulfate, and SO2 (as sulfate) were analyzed directly by ion chromatography. In all cases, the ion chromatographic determination was repeated after 24 h to determine the presence of any dimethyl sulfate in the extract solution by its hydrolysis to monomethyl sulfate ( 3 , 5 ) . Blank runs indicated that sulfate was present in both the nylon, 8.5 f 4.5 nmol/section, and paper, 84 f 22 nmol/section, sections. No diemthyl sulfate or monomethyl sulfate was found in the blank determinations.
Table I. Sampling System Used for the Collection of Gas-Phase Dimethyl Sulfate and Monomethyl Hydrogen Sulfate in the Flue Line of a Coal-Fired Steam Plant sequential parts of sampling system
compounds collected"
species determined
(1)quartz filter
DMS, MSA in particles (CH30)ZSOz, CH30SO;, SO>(2) glass wool plug H2S04,MSA, DMS (CH@)zSOz, CH30S03-,SO>(3) XAD-I1 bed DMS (CH@)zSOz, CH30SO(4) XAD-I1 bed DMS (CH30)2SOz, CH,OSO3-, SO-: (5) nylon filters MSA CH30SO(7) NaOH scrubber SO2 so>"DMS is dimethyl sulfate, and MSA is monomethyl hydrogen sulfate.
Laboratory Studies with Sorbent Materials. The collection of gas-phase dimethyl sulfate, monomethyl hydrogen sulfate, and SO2 by sorbent beds containing XAD-I1 (Rohm and Haas), poly(viny1amine) (PVA), poly(ethy1enimine) (PEI),poly(viny1pyridine) (PVP), or Melamine (21)was studied as a function of temperature from 25 up to 125 "C. The collection of dimethyl sulfate and monomethyl hydrogen sulfate by 47-mm nylon filters was also studied. The gas-phase species were generated in the same manner as described for the denuder experiments. In some cases, two sorbent beds or two nylon filters were placed in series to determine collection efficiency. Any sulfur compounds passing the sorbents were collected in an aqueous bubbler. An acid-washed quartz filter was placed before the sorbent beds to remove any particles. The collected samples were all extracted with water at 25 O C in an ultrasonic bath for 15 min and dimethyl sulfate, monomethyl hydrogen sulfate, sulfate, and SO2determined by ion chromatography as described above. Some XADcollected dimethyl sulfate samples were exposed to ammonia (3) before being extracted with water. In addition, dimethyl sulfate was directly determined in some XAD sorbent samples by direct probe mass spectroscopy. Absorption by Sulfuric Acid Aerosol. Since these analytical techniques are intended to be used in flue line sampling where H2S04 aerosol may be formed if the temperature of the sampled flue line is reduced, the absorption of gas-phase dimethyl sulfate and monomethyl hydrogen sulfate by H2S04(l)was also studied. In these experiments, a quartz filter was wetted with an aqueous H2S04solution and air-dried. Gas-phase dimethyl sulfate and monomethyl hydrogen sulfate were generated in an N2 stream and either filtered and passed over the H2S04-containing filter or mixed with an air stream saturated with H20(g) at 25 "C, filtered, and passed over the H2S04-containing quartz filter. Experiments were preformed with the quartz filter and incoming gas stream at 25 or 75 "C. The moles of gas-phase alkyl sulfate passed through the filter was about 100 times the number of moles of H2S04on the quartz filter. The dimethyl sulfate or monomethyl hydrogen sulfate passing the H2S0, wetted quartz filter was trapped in a H 2 0 bubbler. The bubbler and quartz filter were analyzed for dimethyl sulfate, monomethyl hydrogen sulfate, and sulfuric acid by ion chromatography as described earlier. The acidity of the aqueous extract of the quartz filter was determined by pH measurement. Field Trial Experiments. The collection of gas-phase dimethyl sulfate and monomethyl hydrogen sulfate was tested in field experiments. The configuration of the Environ. Sci. Technol., Vol. 20, No. 9 , 1986
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Table 11. Collection Efficiency of Sorbent Traps for Gas-Phase Dimethyl Sulfate and Monomethyl Hydrogen Sulfate sorbent XAD-I1 PVA PEI PVP Melamine nylon
T, "C 25 60 75 25 25 75 25 75 125 25 75 125 25
efficiency, % (CH30)2S02 CH3OSO3H 99.3 f 1.0 78.7 0.02 0.3 f 0.3 64 i 28 18 19 f 19 0.1 f 0.2
